The limited availability of low-loss integrated inductor structures has long hindered the development of integrated circuits (IC) such as passive filters, voltage controlled oscillators (VCO)s, matching networks, and transformers. Today's portable communications environment strives to achieve more fully integrated circuits that operate at radio frequency (RF) and microwave frequencies. Recent trends indicate a push to integrate entire receivers onto a single substrate.
Most radio receivers are built in bipolar or gallium arsenide (GaAs) technologies, and some integrated planar spiral inductors have been developed which are compatible with widespread silicon-based integrated circuit fabrication processes. However, these planar inductors tend to suffer from high losses and low quality factors (Q) at radio frequencies. These losses and low Q factors are generally attributable to dielectric losses incurred from parasitic capacitances and resistive losses due to the use of thin and relatively high resistivity conductors.
FIG. 1 shows a conventional planar spiral inductor 102 formed on an integrated circuit substrate 104. Typically the planar spiral 102 is formed of a relatively thick copper trace deposited on a silicon substrate. There are several problems that stem from this construction. Firstly, magnetic field lines generated from the RF current flowing in the planar inductor 102 can be represented in the form of closed loops 106 beginning and ending in the center of the planar spiral inductor. The collection of loops 106 forms the shape of a toroid enclosing the spiral inductor 102. As indicated in FIG. 1, the plane containing each individual loop of magnetic field lines is perpendicular to the surface of the integrated planar spiral 102. Thus, the magnetic field lines enter any material above, beside, and below the planar spiral inductor as they complete the closed loop path dictated by Maxwell's equations governing electromagnetics. The observation that the magnetic field lines penetrate the dielectric materials above, beside, and especially below the spiral inductor is very important, because it is in these dielectric materials that losses occur which lower the quality factor of the inductor.
Secondly, unless the spiral inductor is placed far enough away from underlying circuits embedded in the silicon substrate of the IC, the magnetic field generated by the inductor may induce currents in (and therefore disrupt the operation of) underlying integrated circuits. Thirdly, if two planar spirals are placed in close proximity to one another, some degree interaction and cross-coupling will occur between the linked magnetic fields of the two inductors.
Finally, a thin lossy conductor 108 must be located on an underlying metal layer in order to bring a center tap 110 of the spiral inductor 102 to the outside edge of the substrate. This thin underlying conductor 108 adds substantial resistance and parasitic capacitance, along with associated conducted and dielectric losses, which further lower the quality factor of the inductor.
FIG. 2 shows a cross-sectional view of a three-dimensional integrated inductor structure 200 in which copper traces 202 are electroplated around an insulating core 204. However, the insulating core 204 is typically formed of alumina which tends to be too lossy for many high frequency applications, such as multi-GHz VCOs. The use of an insulating core also increases the complexity of the fabrication process and limits the minimum height of the structure to the thickness of a commercially available alumina sheet. Furthermore, the Q factor of these structures is further limited by lossy adhesives 206 generally used in the fabrication process. Hence, it would be desirable from both a design and manufacturing standpoint to eliminate the insulating core while maintaining or increasing the Q factor.
Accordingly, there is a need for a fabrication process which will allow for the implementation of practical integrated RF inductor structures using standard IC processes. A process which provides for the formation of a toroidal and horizontal helical inductor without the use of an insulating core is desirable in order to ease the fabrication process as well as to reduce losses.